In recent years, as a thin type picture display device, a liquid crystal matrix display device, especially a liquid crystal display device of the so-called active matrix type provided with a switching element per each pixel has been under research and development in various places. As a switching element, an MIS type TFT using a-Si is mainly utilized.
FIG. 7 schematically represents an example of the circuit construction of an active matrix type liquid crystal display device using TFT.
In the scanning line 11, when Xi is selected, for example, the gates of respective TFT 13-a connected thereto are turned on all at once, and through the sources of these TFT turned on, the signal voltage corresponding to the picture information is transmitted from respective signal lines 12 to the drains of respective TFT 13-a. A pixel electrode (not shown in the Figure) is connected to the drain, and by the voltage difference between this pixel electrode and the counter electrode 15 formed on the substrate, which is on the other side, when putting the liquid crystal layer 14 between, the optical transparency of the liquid crystal layer 14 is changed to effect a picture image display. When the Xi is in a non-selective state, the gate of each TFT 13-a connected thereto is turned off, and successively, Xi+1 is selected, and the gate of each TFT 13-b is turned on, and a process similar to that as described above is effected. Even after the gate was turned off, since the voltage difference between both the pixel electrode and the counter electrode 15 is preserved by the liquid crystal layer 14 until the same scanning line is selected the next time, the liquid crystal corresponding to each pixel becomes statically driven, and a display of high contrast can be obtained.
As the a-Si TFT used in TFT 13, the TFT produced by the production method having the process of successively depositing a gate insulating layer, an a-Si layer, and a protective insulating layer is hopeful from the viewpoint of reliability, reproducibility of the production method and the like.
FIGS. 8(a) to 8(d) are diagrams schematically representing an example of the production method of the a-Si TFT used in an active matrix type liquid crystal display device, and having the above-described method of production. In the following, explanation will be given by referring to this Figure.
(a) A metal layer of Cr or the like is selectively formed by coating on a glass substrate 21, and a gate electrode 22 and gate wiring (not shown in the Figure) are formed thereon, and successively, a gate insulating layer 23 made of silicon nitride or silicon oxide, an intrinsic a-Si (hereinafter referred to as i-a-Si layer 24 containing almost no impurities which form an active layer, and a protective insulating layer 25 made of silicon nitride or silicon oxide are deposited by using, for example, a plasma CVD method.
(b) The protective insulating layer 25 is selectively etched by use of a buffer fluoric acid solution to expose the i-a-Si layer 24 such as to make a part thereof overlap on the gate electrode 22.
(c) An n-type a-Si (hereinafter referred to as an n-a-Si) layer 26 containing an appropriate amount of phosphorous and a metal layer 27 of Ti or the like are successively deposited, and the metal layer 27 is selectively etched and is patterned into the shape of source and drain electrodes, and using the metal layer 27 and the protective insulating layer 25 as a mask, the n-a-Si layer 26 and i-a-Si layer 24 are etched by use of an organic alkaline solution, and an island-like structure is formed.
(d) A transparent conductive layer 28 of ITO or the like is deposited, and by selectively removing it, a source wiring and a pixel electrode are formed.
By such processes as described above, an a-Si TFT as shown in FIG. 8(d) is completed.
The above-described explanation is mainly related to the production processes of the TFT, but in the peripheral part of the substrate, especially in the end part of the gate wiring, production was carried out by taking notice of the following points. Since the gate wiring requires to effect connection to an external circuit via the connecting terminal of the gate wiring, it must be finally exposed, but for the sake of simplifying the process, a metal mask 32 was arranged to the glass substrate 31 as shown in FIG. 9 to prevent each layer from being deposited on the connecting terminal part of the gate wiring.
When respective layers were deposited by use of a metal mask, not only on the connecting terminals but also on the peripheral glass plate, the respective layers are not deposited. Accordingly, in the TFT forming process described above, in the case of etching the protective insulating layer formed of silicon nitride or silicon oxide, the surface of the glass substrate also becomes etched at the same time. FIG. 10 schematically shows the status of the connecting terminal part at this time. Numeral 21 denotes a glass substrate, numeral 22 a gate wiring, numeral 23 a gate insulating layer, and numeral 24 an i-a-Si layer. In a conventional method, since the surface of the glass substrate is etched, there were such problems that the roughness 29 of the glass surface or the under cut 30 at the end part of the gate wiring 22 occurs. Especially, the under cut 30 causes peeling of the gate wiring 22 or the like, and was a cause of the lowering of the yield of the product.
The present invention has been carried out in view of the above-described defects, and has the main object of removing the roughness of the surface of the glass substrate or the under cut at the end part of the gate wiring or the like.